So the way I read it, it happened like this - these aliens are either basically human themselves, or humanoid enough that mutations actually COULD occur that would result in normal human features and characteristics. (I'm just inferring this so the story will make sense.) Now, they have been viewing Earth through their space-telescopes or whatever, and know what humans look like - therefore, they know what humans look like, and note their resemblance to these mutants of theirs. Now, these aliens are morally advanced enough that they hold no particular prejudice against the mutants, but they wish to have nothing to do with races that would not return the sentiment were the positions reversed. Therefore, they devise a plan - they will arrange contact with Earth, bringing along with them a couple of the mutants. Having landed and ascertained the presence of humans, they will then make ready to lift off, and feed them a cock-and-bull story about mutants of their own. Everything hinges on how the humans react to this - if they respond in a properly enlightened manner, protesting that, hold on a second, these mutants don't sound THAT bad, then proper contact is made, ushering in a mutually benficial contact between two morally equal races, with the mutants acting as ambassadors due to their human form. If, on the other hand, they react with xenophobia and start talking about monsters, then back home the aliens go, leaving the mutants behind to show humankind the error of its ways - and, most likely, since these aliens ARE morally advanced, to give humanity a few hints of their home planet's advanced science, so that the meeting is at least to their advantage in SOME way. Unfortunately for us, we responded in the latter manner.